High-permeability magnetic shield for improved process uniformity in nonmagnetized plasma process chambers

ABSTRACT

A method and apparatus for forming a layer on a substrate in a process chamber during a plasma deposition process are provided. A plasma is formed in a process chamber, a process gas with precursor gases suitable for depositing the layer are flowed into the process chamber, and a magnetic field having a strength less than about 0.5 gauss is attenuated within the process chamber. Attenuation of such a magnetic field results in an improvement in the degree of process uniformity achieved during the deposition.

BACKGROUND OF THE INVENTION

The present invention relates to the manufacture of integrated circuitson a substrate. More particularly, the invention relates to a method andapparatus for improving the process uniformity of plasma processingtechniques used in such manufacture.

One of the primary steps in the fabrication of modem semiconductordevices is the formation of a thin film on a semiconductor substrate bychemical reaction of gases. Such a deposition process is referred togenerally as chemical vapor deposition (“CVD”). Conventional thermal CVDprocesses supply reactive gases to the substrate surface whereheat-induced chemical reactions take place to produce a desired film.Plasma-enhanced CVD (“PECVD”) techniques, on the other hand, promoteexcitation and/or dissociation of the reactant gases by the applicationof radio-frequency (“RF”) energy to a reaction zone near the substratesurface, thereby creating a plasma. The high reactivity of the speciesin the plasma reduces the energy required for a chemical reaction totake place, and thus lowers the temperature required for such CVDprocesses as compared to conventional thermal CVD processes. Theseadvantages are further exploited by high-density-plasma (“HDP”) CVDtechniques, in which a dense plasma is formed at low vacuum pressures sothat the plasma species are even more reactive.

Any of these CVD techniques may be used to deposit conductive orinsulative films as necessary during the fabrication of integratedcircuits. It is generally desirable that the process for depositing sucha film be uniform in all respects. Recently, there has been aneconomically motivated trend to increase the size of circularsemiconductor wafers used in such CVD applications. Currently, waferswith diameters up to 300 mm are being used, up from about 200 mm in therecent past. While the increase in wafer diameter is economicallyadvantageous, it also tends to increase the degree of nonuniformityintroduced during deposition procedures. The effects of suchnonuniformity are especially noticeable when larger wafers are usedbecause the total wafer area varies as the square of its diameter. Inparticular, it has been observed that the sputter nonuniformity in anHDP-CVD process is significantly greater when the process is performedon a 300-mm wafer when compared with the process performed on a 200-mmwafer. Indications suggest that if economic considerations push towardsthe use of even larger wafers, the effects of sputter nonuniformity willbe even greater.

Accordingly, it is desirable to have a method and apparatus that willgenerally improve process uniformity, particularly when larger-sizedwafers are to be used.

SUMMARY OF THE INVENTION

The inventors have discovered that sputter nonuniformity in plasmadeposition processes is affected by magnetic fields on the order of thegeomagnetic field of 0.5 gauss or less. This field can be caused bypermanent magnets in the vicinity of a deposition chamber or by theearth itself. One factor in the sputter nonuniformity is believed toresult from impacts from electrons in the plasma. As wafer sizesincrease so that the diameters exceed the order of the mean cyclotronradius of such electrons, the effect from this factor is enhanced. Sincethe electron cyclotron radius is inversely proportional to the strengthof the ambient magnetic field, attenuation of a magnetic field having astrength less than about 0.5 gauss within the process chamber results inan increase in the cyclotron radius of the electrons, with a concomitantdecrease in the degree of sputter nonuniformity. Accordingly, in a firstembodiment of the invention, a method is provided for forming a layer ona substrate during a plasma deposition process by forming a plasma in aprocess chamber, flowing suitable deposition precursor gases into theprocess chamber, and limiting sputter nonuniformity by attenuating amagnetic field having a strength less than about 0.5 gauss within theprocess chamber.

In specific embodiments of the invention, the attenuation of such amagnetic field is achieved with a magnetic shield positioned to encloseat least a portion of the process chamber. In some of these embodiments,the permeability of the magnetic shield is greater than 10⁴ times thepermeability of free space. In one specific embodiment, an appropriatematerial for the magnetic shield that achieves the desired permeabilitycomprises greater than 75 at. % nickel and greater than 12 at. % iron;it preferably also comprises greater than 4 at. % molybdenum.

The methods of the present invention may be used with a substrateprocessing system. Such a substrate processing system includes anonmagnetized substrate processing chamber and a plasma-generatingsystem operatively coupled to the processing chamber to generate aplasma within the substrate processing chamber. A magnetic shield isconfigured to enclose at least a portion of the process chamber forlimiting sputter nonuniformity by attenuating a magnetic field having astrength less than about 0.5 gauss within the process chamber.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified diagram of one embodiment of a high-densityplasma chemical vapor deposition system according to the presentinvention.

FIG. 1B is a simplified cross section of a gas ring that may be used inconjunction with the exemplary CVD processing chamber of FIG. 1A.

FIG. 1C is a simplified diagram of a monitor and light pen that may beused in conjunction with the exemplary CVD processing chamber of FIG.1A.

FIG. 1D is a flow chart of an exemplary process control computer programproduct used to control the exemplary CVD processing chamber of FIG. 1A;

FIG. 2 shows a cross-sectional view of one embodiment of the inventionin which the magnetic flux leakage of a high-permeability magneticshield is minimized.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS I. INTRODUCTION

Embodiments of the present invention are directed to a method andapparatus for improving the process uniformity during plasma CVDdeposition processes. By enclosing the plasma chamber with a shieldconstructed from a high-magnetic-permeability material, a substantialimprovement in process uniformity, particularly in sputter uniformity,is achieved. As explained in detail below, attenuation of magneticfields on the order of 0.5 gauss or less within the process chamberreduces the sputter nonuniformity, leading to a general improvement indeposition characteristics.

II. EXEMPLARY SUBSTRATE PROCESSING SYSTEM

FIG. 1A illustrates one embodiment of a high density plasma chemicalvapor deposition (HDP-CVD) system 10 in which a dielectric layeraccording to the present invention can be deposited. System 10 includesa chamber 13, a vacuum system 70, a source plasma system 80A, a biasplasma system 80B, a gas delivery system 33, and a remote plasmacleaning system 50.

The upper portion of chamber 13 includes a dome 14, which is made of aceramic dielectric material, such as aluminum oxide or aluminum nitride.Dome 14 defines an upper boundary of a plasma processing region 16.Plasma processing region 16 is bounded on the bottom by the uppersurface of a substrate 17 and a substrate support member 18.

A heater plate 23 and a cold plate 24 surmount, and are thermallycoupled to, dome 14. Heater plate 23 and cold plate 24 allow control ofthe dome temperature to within about ±10° C. over a range of about 100°C. to 200° C. This allows optimizing the dome temperature for thevarious processes. For example, it may be desirable to maintain the domeat a higher temperature for cleaning or etching processes than fordeposition processes. Accurate control of the dome temperature alsoreduces the flake or particle counts in the chamber and improvesadhesion between the deposited layer and the substrate.

The lower portion of chamber 13 includes a body member 22, which joinsthe chamber to the vacuum system. A base portion 21 of substrate supportmember 18 is mounted on, and forms a continuous inner surface with, bodymember 22. Substrates are transferred into and out of chamber 13 by arobot blade (not shown) through an insertion/removal opening (not shown)in the side of chamber 13. Lift pins (not shown) are raised and thenlowered under the control of a motor (also not shown) to move thesubstrate from the robot blade at an upper loading position 57 to alower processing position 56 in which the substrate is placed on asubstrate receiving portion 19 of substrate support member 18. Substratereceiving portion 19 includes an electrostatic chuck 20 that secures thesubstrate to substrate support member 18 during substrate processing. Ina preferred embodiment, substrate support member 18 is made from analuminum oxide or aluminum ceramic material.

Vacuum system 70 includes throttle body 25, which houses twin-bladethrottle valve 26 and is attached to gate valve 27 and turbo-molecularpump 28. It should be noted that throttle body 25 offers minimumobstruction to gas flow, and allows symmetric pumping. Gate valve 27 canisolate pump 28 from throttle body 25, and can also control chamberpressure by restricting the exhaust flow capacity when throttle valve 26is fully open. The arrangement of the throttle valve, gate valve, andturbo-molecular pump allow accurate and stable control of chamberpressures from between about 1 millitorr to about 2 torr.

The source plasma system 80A includes a top coil 29 and side coil 30,mounted on dome 14. A symmetrical ground shield (not shown) reduceselectrical coupling between the coils. Top coil 29 is powered by topsource RF (SRF) generator 31A, whereas side coil 30 is powered by sideSRF generator 31B, allowing independent power levels and frequencies ofoperation for each coil. This dual coil system allows control of theradial ion density in chamber 13, thereby improving plasma uniformity.Side coil 30 and top coil 29 are typically inductively driven, whichdoes not require a complimentary electrode. In a specific embodiment,the top source RF generator 31A provides up to 2,500 watts of RF powerat nominally 2 MHz and the side source RF generator 31B provides up to5,000 watts of RF power at nominally 2 MHz. The operating frequencies ofthe top and side RF generators may be offset from the nominal operatingfrequency (e.g. to 1.7-1.9 MHz and 1.9-2.1 MHz, respectively) to improveplasma-generation efficiency.

A bias plasma system 80B includes a bias RF (“BRF”) generator 31C and abias matching network 32C. The bias plasma system 80B capacitivelycouples substrate portion 17 to body member 22, which act ascomplimentary electrodes. The bias plasma system 80B serves to enhancethe transport of plasma species (e.g., ions) created by the sourceplasma system 80A to the surface of the substrate. In a specificembodiment, bias RF generator provides up to 5,000 watts of RF power at13.56 MHz.

RF generators 31A and 31B include digitally controlled synthesizers andoperate over a frequency range between about 1.8 to about 2.1 MHz. Eachgenerator includes an RF control circuit (not shown) that measuresreflected power from the chamber and coil back to the generator andadjusts the frequency of operation to obtain the lowest reflected power,as understood by a person of ordinary skill in the art. RF generatorsare typically designed to operate into a load with a characteristicimpedance of 50 ohms. RF power may be reflected from loads that have adifferent characteristic impedance than the generator. This can reducepower transferred to the load. Additionally, power reflected from theload back to the generator may overload and damage the generator.Because the impedance of a plasma may range from less than 5 ohms toover 900 ohms, depending on the plasma ion density, among other factors,and because reflected power may be a function of frequency, adjustingthe generator frequency according to the reflected power increases thepower transferred from the RF generator to the plasma and protects thegenerator. Another way to reduce reflected power and improve efficiencyis with a matching network.

Matching networks 32A and 32B match the output impedance of generators31A and 31B with their respective coils 29 and 30. The RF controlcircuit may tune both matching networks by changing the value ofcapacitors within the matching networks to match the generator to theload as the load changes. The RF control circuit may tune a matchingnetwork when the power reflected from the load back to the generatorexceeds a certain limit. One way to provide a constant match, andeffectively disable the RF control circuit from tuning the matchingnetwork, is to set the reflected power limit above any expected value ofreflected power. This may help stabilize a plasma under some conditionsby holding the matching network constant at its most recent condition.

Other measures may also help stabilize a plasma. For example, the RFcontrol circuit can be used to determine the power delivered to the load(plasma) and may increase or decrease the generator output power to keepthe delivered power substantially constant during deposition of a layer.

A gas delivery system 33 provides gases from several sources 34A-34E forprocessing the substrate via gas delivery lines 38 (only some of whichare shown). As would be understood by a person of skill in the art, theactual sources used for sources 34A-34E and the actual connection ofdelivery lines 38 to chamber 13 varies depending on the deposition andcleaning processes executed within chamber 13. Gases are introduced intochamber 13 through a gas ring 37 and/or a top nozzle 45. FIG. 1B is asimplified, partial cross-sectional view of chamber 13 showingadditional details of gas ring 37.

In one embodiment, first and second gas sources, 34A and 34B, and firstand second gas flow controllers, 35A′ and 35B′, provide gas to ringplenum 36 in gas ring 37 via gas delivery lines 38 (only some of whichare shown). Gas ring 37 has a plurality of source gas nozzles 39 (onlyone of which is shown for purposes of illustration) that provide auniform flow of gas over the substrate. Nozzle length and nozzle anglemay be changed to allow tailoring of the uniformity profile and gasutilization efficiency for a particular process within an individualchamber. In a preferred embodiment, gas ring 37 has 12 source gasnozzles made from an aluminum oxide ceramic.

Gas ring 37 also has a plurality of oxidizer gas nozzles 40 (only one ofwhich is shown), which in a preferred embodiment are co-planar with andshorter than source gas nozzles 39, and in one embodiment receive gasfrom body plenum 41. In some embodiments it is desirable not to mixsource gases and oxidizer gases before injecting the gases into chamber13. In other embodiments, oxidizer gas and source gas may be mixed priorto injecting the gases into chamber 13 by providing apertures (notshown) between body plenum 41 and gas ring plenum 36. In one embodiment,third and fourth gas sources, 34C and 34D, and third and fourth gas flowcontrollers, 35C and 35D′, provide gas to body plenum via gas deliverylines 38. Additional valves, such as 43B (other valves not shown), mayshut off gas from the flow controllers to the chamber.

In embodiments where flammable, toxic, or corrosive gases are used, itmay be desirable to eliminate gas remaining in the gas delivery linesafter a deposition. This may be accomplished using a 3-way valve, suchas valve 43B, to isolate chamber 13 from delivery line 38A and to ventdelivery line 38A to vacuum foreline 44, for example. As shown in FIG.1A, other similar valves, such as 43A and 43C, may be incorporated onother gas delivery lines. Such 3-way valves may be placed as close tochamber 13 as practical, to minimize the volume of the unvented gasdelivery line (between the 3-way valve and the chamber). Additionally,two-way (on-off) valves (not shown) may be placed between a mass flowcontroller (“MFC”) and the chamber or between a gas source and an MFC.

Referring again to FIG. 1A, chamber 13 also has top nozzle 45 and topvent 46. Top nozzle 45 and top vent 46 allow independent control of topand side flows of the gases, which improves film uniformity and allowsfine adjustment of the film's deposition and doping parameters. Top vent46 is an annular opening around top nozzle 45. In one embodiment, firstgas source 34A supplies source gas nozzles 39 and top nozzle 45. Sourcenozzle MFC 35A′ controls the amount of gas delivered to source gasnozzles 39 and top nozzle MFC 35A controls the amount of gas deliveredto top gas nozzle 45. Similarly, two MFCs 35B and 35B′ may be used tocontrol the flow of oxygen to both top vent 46 and oxidizer gas nozzles40 from a single source of oxygen, such as source 34B; two MFCs 35D and35D′ may be used to control the flow of the gas provided by source 34Dto source gas nozzles 39 and top gas nozzle 45. The gases supplied totop nozzle 45 and top vent 46 may be kept separate prior to flowing thegases into chamber 13, or the gases may be mixed in top plenum 48 beforethey flow into chamber 13. Separate sources of the same gas may be usedto supply various portions of the chamber.

A remote microwave-generated plasma cleaning system 50 is provided toperiodically clean deposition residues from chamber components. Thecleaning system includes a remote microwave generator 51 that creates aplasma from a cleaning gas source 34E (e.g., molecular fluorine,nitrogen trifluoride, other fluorocarbons or equivalents) receivedthrough MFC 35E in reactor cavity 53. The reactive species resultingfrom this plasma are conveyed to chamber 13 through cleaning gas feedport 54 via applicator tube 55. The materials used to contain thecleaning plasma (e.g., cavity 53 and applicator tube 55) must beresistant to attack by the plasma. The distance between reactor cavity53 and feed port 54 should be kept as short as practical, since theconcentration of desirable plasma species may decline with distance fromreactor cavity 53. Generating the cleaning plasma in a remote cavityallows the use of an efficient microwave generator and does not subjectchamber components to the temperature, radiation, or bombardment of theglow discharge that may be present in a plasma formed in situ.Consequently, relatively sensitive components, such as electrostaticchuck 20, do not need to be covered with a dummy wafer or otherwiseprotected, as may be required with an in situ plasma cleaning process.In one embodiment, this cleaning system is used to dissociate atoms ofthe etchant gas remotely, which are then supplied to the process chamber13. In another embodiment, the etchant gas is provided directly to theprocess chamber 13. In still a further embodiment, multiple processchambers are used, with deposition and etching steps being performed inseparate chambers.

System controller 60 controls the operation of system 10. In a preferredembodiment, controller 60 includes a memory 62, such as a hard diskdrive, a floppy disk drive (not shown), and a card rack (not shown)coupled to a processor 61. The card rack may contain a single-boardcomputer (SBC) (not shown), analog and digital input/output boards (notshown), interface boards (not shown), and stepper motor controllerboards (not shown). The system controller conforms to the Versa ModularEuropean (“VME”) standard, which defines board, card cage, and connectordimensions and types. The VME standard also defines the bus structure ashaving a 16-bit data bus and 24-bit address bus. System controller 60operates under the control of a computer program stored on the hard diskdrive or through other computer programs, such as programs stored on aremovable disk. The computer program dictates, for example, the timing,mixture of gases, RF power levels and other parameters of a particularprocess. The interface between a user and the system controller is via amonitor, such as a cathode ray tube (“CRT”) 65, and a light pen 66, asdepicted in FIG. 1C.

FIG. 1C is an illustration of a portion of an exemplary system userinterface used in conjunction with the exemplary CVD processing chamberof FIG. 1A. System controller 60 includes a processor 61 coupled to acomputer-readable memory 62. Preferably, memory 62 may be a hard diskdrive, but memory 62 may be other kinds of memory, such as ROM, PROM,and others.

System controller 60 operates under the control of a computer program 63stored in a computer-readable format within memory 62. The computerprogram dictates the timing, temperatures, gas flows, RF power levelsand other parameters of a particular process. The interface between auser and the system controller is via a CRT monitor 65 and a light pen66, as depicted in FIG. 1C. In a preferred embodiment, two monitors, 65and 65A, and two light pens, 66 and 66A, are used, one mounted in theclean room wall (65) for the operators and the other behind the wall(65A) for the service technicians. Both monitors simultaneously displaythe same information, but only one light pen (e.g. 66) is enabled. Toselect a particular screen or function, the operator touches an area ofthe display screen and pushes a button (not shown) on the pen. Thetouched area confirms being selected by the light pen by changing itscolor or displaying a new menu, for example.

The computer program code can be written in any conventionalcomputer-readable programming language such as 68000 assembly language,C, C++, or Pascal. Suitable program code is entered into a single file,or multiple files, using a conventional text editor and is stored orembodied in a computer-usable medium, such as a memory system of thecomputer. If the entered code text is in a high level language, the codeis compiled, and the resultant compiler code is then linked with anobject code of precompiled windows library routines. To execute thelinked compiled object code, the system user invokes the object codecausing the computer system to load the code in memory. The CPU readsthe code from memory and executes the code to perform the tasksidentified in the program.

FIG. 1D shows an illustrative block diagram of the hierarchical controlstructure of computer program 100. A user enters a process set numberand process chamber number into a process selector subroutine 110 inresponse to menus or screens displayed on the CRT monitor by using thelight pen interface. The process sets are predetermined sets of processparameters necessary to carry out specified processes, and areidentified by predefined set numbers. Process selector subroutine 110identifies (i) the desired process chamber in a multichamber system, and(ii) the desired set of process parameters needed to operate the processchamber for performing the desired process. The process parameters forperforming a specific process relate to conditions such as process gascomposition and flow rates, temperature, pressure, plasma conditionssuch as RF power levels, and chamber dome temperature, and are providedto the user in the form of a recipe. The parameters specified by therecipe are entered utilizing the light pen/CRT monitor interface.

The signals for monitoring the process are provided by the analog anddigital input boards of system controller 60, and the signals forcontrolling the process are output on the analog and digital outputboards of system controller 60.

A process sequencer subroutine 120 comprises program code for acceptingthe identified process chamber and set of process parameters from theprocess selector subroutine 110 and for controlling operation of thevarious process chambers. Multiple users can enter process set numbersand process chamber numbers, or a single user can enter multiple processset numbers and process chamber numbers; sequencer subroutine 120schedules the selected processes in the desired sequence. Preferably,sequencer subroutine 120 includes a program code to perform the steps of(i) monitoring the operation of the process chambers to determine if thechambers are being used, (ii) determining what processes are beingcarried out in the chambers being used, and (iii) executing the desiredprocess based on availability of a process chamber and type of processto be carried out. Conventional methods of monitoring the processchambers can be used, such as polling. When scheduling which process isto be executed, sequencer subroutine 120 can be designed to take intoconsideration the “age” of each particular user-entered request, or thepresent condition of the process chamber being used in comparison withthe desired process conditions for a selected process, or any otherrelevant factor a system programmer desires to include for determiningscheduling priorities.

After sequencer subroutine 120 determines which process chamber andprocess set combination is going to be executed next, sequencersubroutine 120 initiates execution of the process set by passing theparticular process set parameters to a chamber manager subroutine130A-C, which controls multiple processing tasks in chamber 13 andpossibly other chambers (not shown) according to the process set sent bysequencer subroutine 120.

Examples of chamber component subroutines are substrate positioningsubroutine 140, process gas control subroutine 150, pressure controlsubroutine 160, and plasma control subroutine 170. Those having ordinaryskill in the art will recognize that other chamber control subroutinescan be included depending on what processes are selected to be performedin chamber 13. In operation, chamber manager subroutine 130A selectivelyschedules or calls the process component subroutines in accordance withthe particular process set being executed. Chamber manager subroutine 130A schedules process component subroutines in the same manner thatsequencer subroutine 120 schedules the process chamber and process setto execute. Typically, chamber manager subroutine 130A includes steps ofmonitoring the various chamber components, determining which componentsneed to be operated based on the process parameters for the process setto be executed, and causing execution of a chamber component subroutineresponsive to the monitoring and determining steps.

Operation of particular chamber component subroutines will now bedescribed with reference to FIGS. 1A and 1D. Substrate positioningsubroutine 140 comprises program code for controlling chamber componentsthat are used to load a substrate onto substrate support number 18.Substrate positioning subroutine 140 may also control transfer of asubstrate into chamber 13 from, e.g., a plasma-enhanced CVD (“PECVD”)reactor or other reactor in the multi-chamber system, after otherprocessing has been completed.

Process gas control subroutine 150 has program code for controllingprocess gas composition and flow rates. Subroutine 150 controls theopen/close position of the safety shut-off valves and also rampsup/ramps down the mass flow controllers to obtain the desired gas flowrates. All chamber component subroutines, including process gas controlsubroutine 150, are invoked by chamber manager subroutine 130A.Subroutine 150 receives process parameters from chamber managersubroutine 130A related to the desired gas flow rates.

Typically, process gas control subroutine 150 opens the gas supplylines, and repeatedly (i) reads the necessary mass flow controllers,(ii) compares the readings to the desired flow rates received fromchamber manager subroutine 130A, and (iii) adjusts the flow rates of thegas supply lines as necessary. Furthermore, process gas controlsubroutine 150 may include steps for monitoring the gas flow rates forunsafe rates and for activating the safety shut-off valves when anunsafe condition is detected.

In some processes, an inert gas, such as argon, is flowed into chamber13 to stabilize the pressure in the chamber before reactive processgases are introduced. For these processes, the process gas controlsubroutine 150 is programmed to include steps for flowing the inert gasinto chamber 13 for an amount of time necessary to stabilize thepressure in the chamber. The steps described above may then be carriedout.

Additionally, when a process gas is to be vaporized from a liquidprecursor, for example, tetraethylorthosilane (TEOS), the process gascontrol subroutine 150 may include steps for bubbling a delivery gassuch as helium through the liquid precursor in a bubbler assembly or forintroducing the helium to a liquid injection valve. For this type ofprocess, the process gas control subroutine 150 regulates the flow ofthe delivery gas, the pressure in the bubbler, and the bubblertemperature to obtain the desired process gas flow rates. As discussedabove, the desired process gas flow rates are transferred to process gascontrol subroutine 150 as process parameters.

Furthermore, the process gas control subroutine 150 includes steps forobtaining the necessary delivery gas flow rate, bubbler pressure, andbubbler temperature for the desired process gas flow rate by accessing astored table containing the necessary values for a given process gasflow rate. Once the necessary values are obtained, the delivery gas flowrate, bubbler pressure and bubbler temperature are monitored, comparedto the necessary values and adjusted accordingly.

The process gas control subroutine 150 may also control the flow ofheat-transfer gas, such as helium (He), through the inner and outerpassages in the wafer chuck with an independent helium control (IHC)subroutine (not shown). The gas flow thermally couples the substrate tothe chuck. In a typical process, the wafer is heated by the plasma andthe chemical reactions that form the layer, and the He cools thesubstrate through the chuck, which may be water-cooled. This keeps thesubstrate below a temperature that may damage preexisting features onthe substrate.

Pressure control subroutine 160 includes program code for controllingthe pressure in chamber 13 by regulating the size of the opening ofthrottle valve 26 in the exhaust portion of the chamber. There are atleast two basic methods of controlling the chamber with the throttlevalve. The first method relies on characterizing the chamber pressure asit relates to, among other things, the total process gas flow, the sizeof the process chamber, and the pumping capacity. The first method setsthrottle valve 26 to a fixed position. Setting throttle valve 26 to afixed position may eventually result in a steady-state pressure.

Alternatively, the chamber pressure may be measured, with a manometerfor example, and the position of throttle valve 26 may be adjustedaccording to pressure control subroutine 360, assuming the control pointis within the boundaries set by gas flows and exhaust capacity. Theformer method may result in quicker chamber pressure changes, as themeasurements, comparisons, and calculations associated with the lattermethod are not invoked. The former method may be desirable where precisecontrol of the chamber pressure is not required, whereas the lattermethod may be desirable where an accurate, repeatable, and stablepressure is desired, such as during the deposition of a layer.

When pressure control subroutine 160 is invoked, the desired, or target,pressure level is received as a parameter from chamber managersubroutine 130A. Pressure control subroutine 160 measures the pressurein chamber 13 by reading one or more conventional pressure manometersconnected to the chamber; compares the measured value(s) to the targetpressure; obtains proportional, integral, and differential (PID) valuesfrom a stored pressure table corresponding to the target pressure, andadjusts throttle valve 26 according to the PID values obtained from thepressure table. Alternatively, pressure control subroutine 160 may openor close throttle valve 26 to a particular opening size to regulate thepressure in chamber 13 to a desired pressure or pressure range.

Plasma control subroutine 170 comprises program code for controlling thefrequency and power output setting of RF generators 31A and 31B and fortuning matching networks 32A and 32B. Plasma control subroutine 170,like the previously described chamber component subroutines, is invokedby chamber manager subroutine 130A.

An example of a system that may incorporate some or all of thesubsystems and routines described above would be the ULTIMA™ system,manufactured by APPLIED MATERIALS, INC., of Santa Clara, Calif.,configured to practice the present invention. Further details of such asystem are disclosed in commonly assigned U.S. Pat. No. 6,170,428, filedJul. 15, 1996, entitled “Symmetric Tunable Inductively-Coupled HDP-CVDReactor,” having Fred C. Redeker, Farhad Moghadam, Hirogi Hanawa,Tetsuya Ishikawa, Dan Maydan, Shijian Li, Brian Lue, Robert Steger,Yaxin Wang, Manus Wong and Ashok Sinha listed as co-inventors, thedisclosure of which is incorporated herein by reference. The describedsystem is for exemplary purpose only. It would be a matter of routineskill for a person of skill in the art to select an appropriateconventional substrate processing system and computer control system toimplement the present invention.

III. MAGNETIC SHIELDING

In response to recent trends towards the use of larger semiconductorwafers, the inventors were tasked with developing deposition processesfor chambers to accommodate 300-mm wafers. During this development forplasma-based processes, the inventors were faced with unexpectedly largefilm uniformity problems. Such problems were encountered independent ofthe material being deposited and were observed, for example, whendepositing undoped silicate glass (USG) or fluorinate silicate glass(FSG).

Over a period of time, various approaches that had been successful inimproving the process uniformity for 200-mm wafers were attempted,including adjusting the deposition parameters and bias characteristicsof the process. While various of these approaches had some effect, theygenerally affected the process uniformity only to the degree expectedfrom previous experience with 200-mm wafers and were unable to correctthe anomalous nonuniformity seen with 300-mm wafers. After excludingthese various approaches, the inventors hypothesized that the presenceof a spurious magnetic field might be adversely affecting the processcharacteristics. They therefore sought to confirm this hypothesis byremoving magnetic sources from the vicinity of the process 300-mmprocess chamber, such magnetic sources generally having a field strengthgreater than about 0.5 gauss, which is of the order of the geomagneticfield. Even after carefully excluding magnetic sources potentiallyhaving producing fields greater than about 0.5 gauss, however, theanomalous nonuniformity nevertheless persisted.

After further effort, the inventors therefore theorized that the anomalymight arise from a magnetic source having a field strength less thanabout 0.5 gauss. For the reasons expressed below on the basis ofprocessing smaller wafers, such field strengths were believed to besufficiently small not to have a significant effect on processcharacteristics. It was further hypothesized that the anomaly mightresult from an ambient magnetic field that could not be excluded byremoving specific field sources from the area of the 300-mm processchamber. In order to test these theories, magnetic shielding was use toisolate the process chamber 13 even from an ambient magnetic field andproduced a substantial improvement, sufficient to account for theanomalous nonuniformity. The use of magnetic shielding on plasmadeposition chambers had previously been restricted to reducing theeffect of purely local magnetic fields produced by known permanentmagnets having field strengths greater than about 0.5 gauss in thevicinity of the process chamber 13. The inventors' discovery is that,even in the absence of locally induced magnetic fields with suchstrengths, process uniformity is improved by configuring ahigh-magnetic-permeability shield around a nonmagnetic process chamber.This suggests that the spurious nonuniformity is manifested with 300-mmwafers for smaller fields than was the case for smaller-diameter wafers.These field strengths may be caused by permanent magnets in the vicinityof the chamber or by the earth itself.

There are several subsequently developed experimental and theoreticalconsiderations that support this hypothesis. During a plasma depositionprocess, sputter nonuniformity during the deposition of a layer on asubstrate arises from impacts by electrons in the plasma. The cyclotronradius for an electron moving in a magnetic field with strength B isgiven by r_(cyc)=mv/eB, where m and e are the electron mass and charge,and v is the electron's velocity. Under normal deposition conditions forthe plasma chamber 13 described above, the plasma species includes amixture of electrons and ionic particles, each of which has an energydistribution. Electrons with a mean expected kinetic energy on the orderof 5 eV thus have a cyclotron radius on the order of 100 mm when thefield strength is on the order of 0.5 gauss.

It is thus evident that while the effect on process uniformity of fieldshaving a strength on the order of 0.5 gauss or less should be small forwafers with a diameter less than this cyclotron radius (i.e. ford_(wafer)≦100 mm), the effect is increased for wafers with largerdiameters. It is believed this is why the process exhibited significantnonuniformity for 300-mm wafers, but was not previously recognized forthe smaller 100-mm and 200-mm wafers. The ionic particles in the plasmahave a much larger mass than the electrons, and their cyclotron radiusis therefore expected to be thousands of times larger. As a result, theyhave little effect on the sputter uniformity. While the abovedescription of the sputter nonuniformity mechanism has focussed onhigh-density plasma deposition, for which “high density” is understoodto refer to a plasma with an ion density exceeding 10¹¹ ions/cm³, theresult that the sputter nonuniformity mechanism is dominated by electronactivity is generally applicable to any plasma deposition process.

This mechanism has been confirmed qualitatively with experiments inwhich a small magnetization was deliberately introduced to the plasma.The introduction of a magnetization that corresponds to what wouldresult from field strengths on the order of 0.5 gauss was observed toproduce noticeable effects on sputtering uniformity when depositinglayers on 300-mm wafers in an HDP-CVD process chamber. With carefulobservation, the effect could also be seen when depositing layers on200-mm wafers, but was significantly smaller. It is thus apparent thatthe influence of magnetic fields less than about 0.5 gauss should beaddressed in order to maintain desired process uniformitycharacteristics as wafer sizes are increased above 200 mm. In terms ofthe sputter mechanism described above, attenuation of such small fieldsresults in an increase in the cyclotron radius of the plasma electrons.When the mean electron cyclotron radius approximately exceeds the waferdiameter, the sputter nonuniformity decreases.

In one embodiment of the invention, attenuation of fields less than 0.5gauss within the process chamber 13 is achieved by shielding thenonmagnetic process chamber 13 with high-magnetic-permeability μ sheetmetal. The magnetic permeability of a metal is understood to refer tothe ratio of magnetic flux induced in the metal to the strength of themagnetic field that induces that flux. Accordingly, a shield's highpermeability ensures that magnetic flux will be concentrated in theshield rather than within the nonmagnetic process chamber 13, therebyachieving the desired attenuation. The shielding enclosure is preferablyconstructed to surround as much of the process chamber 13 as possible,but outside of any RF and/or ground shields that may be used as part ofthe substrate processing system 10, so as not to affect the RF fieldswithin the chamber. As discussed below, the most effective shielding isone that encloses as much of the process chamber 13 as possible, butpartial shields have also been observed to have a favorable effect onthe process uniformity.

When a high-μ material is placed in a magnetic field, the local magneticflux is diverted to the material, causing the desired reduction in fieldstrength. Because the extent to which flux is diverted is proportionalto the permeability of the material, continually greater improvement inshielding results with an increase in the permeability of the shieldingmaterial. Materials that have suitably high permeabilities to shield theprocess chamber 13 from the magnetic fields on the order of 0.5 gauss orless include MUMETAL®, HIPERNOM®, CARPENTER HyMu-80®, and PERMALLOY®,although any material with an appropriately high permeability may beused. Each of these four commercially available materials is a softalloy that has a permeability relative to the permeability of free spaceon the order of 10⁴-10⁶; they comprise approximately 80 at. % Ni and 15at. % Fe, and are balanced primarily with transition elements such ascopper, molybdenum, or chromium, depending on the specific recipe used.For example, MUMETAL® consists of 77 at. % Ni, 14 at. % Fe, 5 at. % Cu,and 4 at. % Mo. It has a magnetic permeability between approximately6.0×10⁴ and 2.4×10⁵, depending on the frequency of the magnetic field inwhich it is placed. The CARPENTER HyMu-80® alloy consists of 80 at. %Ni, 15 at. % Fe, 4.2 at. % Mo, 0.5 at. % Mn, 0.35 at. % Si, and 0.02 at.% C. PERMALLOY® consists of 78 at. % Ni, 16.6 at. % Fe, 4.8 at. % Mo,and 0.9 at. % Mn.

As will be appreciated by those of skill in the art, such shieldingmaterials are substantially different from ground or RF shields that mayalso be used within the substrate processing system 10. RF shielding isused to block high-frequency (≧100 kHz) interference fields. Suchshields are typically constructed of copper, aluminum, galvanized steel,or conductive rubber, plastic, or paints. The high electricalconductivity of such materials, with small (˜1) permeabilities, makesthem suitable for blocking electromagnetic signals at high frequency.Accordingly, by positioning the high-permeability shield outside of RFand/or ground shields, the operation of the substrate processing system10 is not impeded in any way as a result of field attenuation fromexternal or ambient fields having a strength ≦0.5 gauss.

Experimental observations of 300-mm wafers deposited with layers whilehigh-permeability magnetic shielding is in place confirm directly thatthe sputter nonuniformity is decreased. Careful observations ofdeposited 200-mm wafers also reveal a beneficial effect from theshielding, although, as expected, the effect is less significant thanfor the larger wafers. In constructing the high-permeability shield, itis preferable to use a shielding configuration that affords a completepath for the field lines; otherwise there is the possibility that thefield lines will exit the material in a place where they will causeunintended and undesirable interference with the operation of thesubstrate processing system 10. The shape and configuration of thesubstrate processing system 10 may impose limitations on the extent towhich the process chamber 13 can be enclosed by the magnetic shield, butin order to attenuate the fields less than about 0.5 gauss as much aspossible, it is preferable to enclose as much of the process chamber 13as practicable. Less attenuation of such fields permits more plasmaelectrons at the low end of their energy distribution to have asufficiently small cyclotron radius to affect sputter uniformity. Evenif the configuration of the substrate processing system 10 preventscomplete enclosure of the process chamber 13, however, partial shieldingis still observed to have a favorable effect on process uniformitybecause it limits the portion of the plasma electron energy distributionthat can have an effect.

An effective shield that limits the escape of flux can be formed byjoining plates of the high-permeability material tightly, minimizinggaps between the plates. One useful configuration is illustrated in FIG.2. A small angled piece 210 of high-permeability material is positionedto ensure that joined plates 220 and 230 have some overlap. Suchpositioning helps ensure that the magnetic field lines will not leak tothe space enclosed by the shield. The possibility of such undesirableleakage is further decreased by welding the plates 220 and 230 to theangled piece 210.

Those of ordinary skill in the art will realize that the material usedto shield the process chamber may have different compositions and may beconfigured differently without departing from the spirit of theinvention. Other variations will also be apparent to persons of skill inthe art. These equivalents and alternatives are intended to be includedwithin the scope of the present invention. Therefore, the scope of thisinvention should not be limited to the embodiments described, but shouldinstead be defined by the following claims.

What is claimed is:
 1. A method for forming a layer on a substrate in anonmagnetized process chamber during a plasma deposition process, themethod comprising: (a) forming a plasma in the process chamber; (b)flowing a process gas suitable for depositing the layer on the substrateinto the process chamber; and (c) limiting sputter nonuniformity byattenuating a magnetic field having a strength less than about 0.5 gausswithin the process chamber with a magnetic shield that at leastpartially encloses the process chamber.
 2. The method according to claim1 wherein the plasma is a high-density plasma.
 3. The method accordingto claim 1 wherein the substrate is a circular wafer with a diametergreater than 200 mm.
 4. The method according to claim 3 wherein thediameter of the circular wafer is substantially equal to 300 mm.
 5. Themethod according to claim 1 wherein the magnetic shield has a magneticpermeability greater than 10⁴ times the magnetic permeability of freespace.
 6. The method according to claim 1 wherein the magnetic shieldcomprises greater than 75 at. % Ni and greater than 12 at. % Fe.
 7. Themethod according to claim 6 wherein the magnetic shield furthercomprises greater than 4 at. % Mo.
 8. The method according to claim 1wherein the magnetic shield encloses substantially all of the processchamber.
 9. The method according to claim 1 wherein the magnetic fieldis a geomagnetic field.
 10. A method for forming a layer on a circularwafer having a diameter greater than 200 mm in a nonmagnetized processchamber during a high-density-plasma deposition process, the methodcomprising: (a) forming a plasma in the process chamber; (b) flowing aprocess gas suitable for depositing the layer on the substrate into theprocess chamber; and (c) limiting sputter nonuniformity by attenuating amagnetic field having a strength less than about 0.5 gauss within theprocess chamber with a magnetic shield having a magnetic permeabilitygreater than 10⁴ times the magnetic permeability of free space.
 11. Themethod according to claim 10 wherein the magnetic shield comprisesgreater than 75 at. % Ni, greater than 12 at. % Fe, and greater than 4at. % Mo.
 12. The method according to claim 10 wherein the diameter ofthe circular wafer is approximately 300 mm or greater.
 13. The methodaccording to claim 10 wherein the magnetic field is a geomagnetic field.14. A substrate processing system comprising: (a) a nonmagnetizedsubstrate processing chamber; (b) a plasma-generating system operativelycoupled to the processing chamber to generate a plasma within thesubstrate processing chamber; and (c) a magnetic shield configured toenclose at least a portion of the process chamber for limiting sputternonuniformity by attenuating a magnetic field having a field strengthless than about 0.5 gauss within the process chamber.
 15. The substrateprocessing system according to claim 14 wherein the substrate processingchamber is sized and configured to hold a circular wafer with a diametergreater than 200 mm.
 16. The substrate processing system according toclaim 15 wherein the substrate processing chamber is sized andconfigured to hold a circular wafer with a diameter substantially equalto 300 mm.
 17. The substrate processing system according to claim 14wherein the plasma is a high-density plasma.
 18. The substrateprocessing system according to claim 14 wherein the magnetic shield hasa magnetic permeability greater than 10⁴ times the magnetic permeabilityof flee space.
 19. The substrate processing system according to claim 14wherein the magnetic shield comprises greater than 75 at. % Ni andgreater than 12 at. % Fe.
 20. The substrate processing system accordingto claim 19 wherein the magnetic shield further comprises greater than 4at. % Mo.
 21. The substrate processing system according to claim 14wherein the magnetic shield encloses substantially all of the processchamber.
 22. A substrate processing system comprising: (a) anonmagnetized substrate processing chamber sized and configured to holda circular wafer having a diameter greater than 200 mm; (b) ahigh-density plasma generating system operatively coupled to theprocessing chamber to generate a plasma within the substrate processingchamber; and (c) a magnetic shield having a magnetic permeabilitygreater than 10⁴ times the magnetic permeability of free space andconfigured to enclose at least a portion of the process chamber forlimiting sputter nonuniformity by attenuating a magnetic field having afield strength less than about 0.5 gauss within the process chamber. 23.The substrate processing system according to claim 22 wherein themagnetic shield comprises greater than 75 at. % Ni, greater than 12 at.% Fe, and greater than 4 at. % Mo.
 24. The substrate processing systemaccording to claim 22 wherein the diameter of the circular wafer isapproximately 300 mm or greater.